This invention concerns the application of substantially amorphous fluoropolymer compositions to stone in order to protect the stone from the deleterious effects of water and pollution. This invention is useful for the preservation of historic monuments, buildings, and sculptures and for the construction of weather and pollution resistant stone construction materials.
It has long been recognized that a combination of man-made and natural atmospheric conditions are having deleterious effects on stone structures including many monuments of considerable artistic and historical importance. A variety of efforts have been undertaken over the years to identify ways to protect and preserve these structures, but these efforts have met with only partial success. Most recently, a well-organized and concerted effort has been undertaken by Piacenti and coworkers, with sponsorship from the Target Project for the Cultural Heritage of the Consiglio Nazionale della Recerche of Rome, Italy, and represents the current state of the art.
The deleterious effects of water, in the form of both rainfall and condensation, are of primary concern, although organic pollutants are also of concern. For example, all building materials are subject to stress and concomitant cracking resulting from penetration by water followed by cycles of freezing and thawing. Also, water in combination with CO2, which occurs naturally, and nitrous and sulfurous gases, which are man-made pollutants, forms acids which rapidly eat away at the stone.
A successful attack on the problem will necessitate some tradeoffs. While it is highly desirable to minimize the contact between water and stone, by achieving maximum water repellency, it is also necessary to provide high water vapor permeability in order to permit venting of that water which finds its way into the microstructure of the stone. Substances with high permeability to water vapor are often not those of the highest water repellency. High resistance to acid and abrasion are also of considerable importance. Furthermore, cost of materials is a factor in any practical application. And, for several reasons, the smaller the amount of material required to achieve the desired effect, the better.
There are other tradeoffs. For example, it is particularly desirable that the coating material coat but preferably not block the pores. To achieve this, a coating viscosity should be in a range which permits wetting of the pores via capillary action. High wetting is also required to ensure thorough and uniform coverage. However, the coating must be provided with sufficient adhesion to the outside surface upon which it is deposited that at least some amount will remain thereon.
Other requirements for such materials include chemical inertness, low volatility, photooxidative stability and thermal stability. There should also be sufficient solubility in environmentally friendly solvents for the purposes of both application and subsequent removal if warranted. The coating must also be clear and colorless, and remain so for its lifetime. In the current state of the art, the application solvent of choice is supercritical CO2, as described in Carbonell et al., WO 99/19080.
In a series of patents, U.S. Pat. No. 4,499,146, U.S. Pat. No. 4,746,550, U.S. Pat. No. 4,745,009, U.S. Pat. No. 4,902,538, Piacenti et al. disclose compositions based upon perfluoropolyethers having molecular weights in the range of 500-5000 for use in the protection of stone from the effects of water and atmospheric pollutants. In the art of Piacenti, excellent combinations of water repellency and water vapor permeability are achieved.
In U.S. Pat. No. 4,902,538, good results are achieved in compositions having highly crystalline particles of polytetrafluoroethylene and copolymers thereof intermixed with the perfluoropolyethers. However, when stone of porosity of greater than ca. 30% is treated, impractically high levels of coating material are required to achieve the desired coverage with the desired water repellency. Levels in the range of at least 150 g/m2 are disclosed, more than 10 times the amount required for low-porosity marble. The effect of this high coating level on permeability is not disclosed. Its effect on cost, however, is clearly undesirable. Furthermore, use of highly crystalline polymers, such as polytetrafluoroethylene, is undesirable because, unless they are sintered at high temperatures, they will be too readily susceptible to removal from the treated surface by abrasion and erosion. Further still, they are not readily soluble in the delivery medium of choice, CO2, or any other desirable medium.
Also disclosed in the art in Piacenti et al., U.S. Pat. No. 4,764,431, are copolymers of vinylidene fluoride which are less effective than the perfluoropolyethers.
Fluorinated acrylic polymers are disclosed by Ciardelli et al., Prog. in Org. Coatings, 32, 43-50 (1997). The polymers disclosed therein are characterized by hydrocarbon backbones and fluorinated pendant groups. These polymers exhibit similar functionality to the perfluoropolyethers.
Guidetti et al. disclose the use of polyfluorosubstantially amorphous fluoropolymers for protecting stone in xe2x80x9cPolyfluorosubstantially amorphous fluoropolymers as stone protectivesxe2x80x9d, 7th International Congress on Deterioration and Conservation of Stone, 1053-62 (1992).
There is considerable incentive in the art to discover new materials which possess several of the above attributes desired for the application.
The present invention provides a process for protecting stone comprising:
contacting stone with a substantially amorphous fluoropolymer comprising
(a) at least about 10 mole percent of repeat units of the formula
xe2x80x94CF2xe2x80x94CF(CF3)xe2x80x94xe2x80x83xe2x80x83(I); 
(b) 0 to 50 mole percent of repeat units of the formula
xe2x80x94CF2xe2x80x94CF2xe2x80x94xe2x80x83xe2x80x83(II); 
xe2x80x83and
(c) about 10 to about 75 mol-% of repeat units selected from the group consisting of:
xe2x80x94CF2CFXxe2x80x94, xe2x80x94CH2xe2x80x94CHRxe2x80x94, xe2x80x94CH2CF2xe2x80x94xe2x80x83xe2x80x83(III) 
and mixtures thereof wherein X is H or perfluoroalkoxy having 1-20 carbons, and R is H, alkyl, alkoxy, perfluoroalkyl, F. Furthermore, the backbone H:F ratio should be about 0.3 to about 1.0. If the polymer MW is less than about 5000, the H:F ratio can be broader.
For the purpose of the present invention, the term xe2x80x9cstonexe2x80x9d means a natural stone used in construction or sculpture (such as granite, marble, limestone, or sandstone) as well as tile, cement, brick, stucco, and the like.
The method of the present invention provides surprising benefits over the methods of the art. In the method of the present invention, a substantially amorphous fluoropolymer composition is employed as a coating agent for stone in order to provide high liquid moisture barrier, good moisture vapor permeability, and resistance to environmental pollutants. The non-fugitive, very low areal density coating formed on the stone surface is surprisingly effective over the materials of the art. Furthermore, the substantially amorphous fluoropolymer of the present invention is readily soluble in a variety of solvents by virtue of its amorphous nature, and is thereby both readily applied in the form of an environmentally friendly solution and readily removed by conventional solvents should that be deemed necessary after application. Further still, the highly desirable effects of the method of the present invention are achieved employing a substantially amorphous fluoropolymer in relatively small quantities in order to achieve the desired combination of water vapor permeability and liquid water resistance.
The polymers suitable for use in the present invention are substantially amorphous, in contrast to most fluorinated polymers in common use which are known to be moderately to highly crystalline. One of skill in the art will appreciate that the degree of polymer crystallinity which can be tolerated in a given situation will depend upon the specific polymer structure, solvents, other adjuvants, application methods, requisites of the particular application, and substrate in a given practical embodiment of the invention. For the purpose of the present invention, amorphous polymers suitable for the practice of the invention may exhibit a melting endotherm having an associated heat of fusion no greater than 5 J/g, preferably no greater than 2 J/g, more preferably no greater than 1 J/g, at a temperature above about 20xc2x0 C.
More preferably the polymers employed for the practice of the invention will exhibit no melting endotherm above about 100xc2x0 C. Most preferably the polymers employed for the practice of the invention will exhibit no melting endotherm whatever.
For the purpose of the present invention, the heat of fusion is determined by differential scanning calorimetry (DSC) at a heating rate of 10xc2x0 C./min, according to ASTM D4591-97.
DSC is also the technique of choice for determining the glass transition temperature. Glass transition temperatures of the polymer are preferably no higher than 30xc2x0 C., most preferably no higher than 20xc2x0 C. Glass transitions should be set by the methods herein described so that the polymer will not undergo repeated transitions while in place on the stone.
The polymers suitable for use in the present invention may be made via a continuous polymerization process, for example according to the teachings of Anolick et al., U.S. Pat. No. 5,663,255. Continuous polymerization reactors include continuous stirred tank reactors and pipeline (tubular) reactors, both of which are well-known in the art. The process is run at a pressure of about 41 to about 690 MPa, especially about 69 to about 103 MPa. At lower pressures the molecular weight of the polymers formed and the conversion of monomers to polymer both tend to decrease. Solvents can be used in the reactor. so that a polymer solution may be made in a single step. When solvents are used it is preferred that they be essentially inert under process conditions. Useful solvents include perfluorodimethylcyclobutane and perfluoro(n-butyltetrahydrofuran). A particularly useful solvent is CO2.
The polymer is soluble in the monomer(s) under the process conditions. Therefore, one method of polymer isolation is to reduce the pressure below that required for solution of the polymer, and isolate the polymer from that, as by decantation, filtration or centrifugation.
The apparatus for running the polymerization may be any suitable pressure apparatus in which the reactant and products streams may be added and removed at appropriate rates. Thus the apparatus may be a stirred or unstirred autoclave, a pipeline type reactor, or other suitable apparatus. Agitation is not necessary, but preferable, especially to obtain polymers with low polydispersity. The material of construction should be suitable for the process ingredients, and metals such as stainless steel are often suitable.
The polymerization is carried out above about 200xc2x0 C., and most preferably from about 250xc2x0 to about 400xc2x0 C. The initiator is chosen so that it will generate active free radicals at the temperature at which the polymerization is carried out. Such free radical sources, particularly those suitable for hydrocarbon vinyl monomers at much lower temperatures, are known to one of skill in the art, see for instance J. Brandrup, et al., Ed., Polymer Handbook, 3rd Ed., John Wiley and Sons, New York, 1989, p. II/1 to II/65. The preferred temperature for running the instant process depends on both the monomers and the initiator and is often a compromise between raising temperature to favor high productivities and high conversions and lowering temperature to minimize chain transfer and monomer degradation. For the copolymerization of HFP with TFE, for example, where chain transfer is not a problem, initiation by C2F5SO2 C2F5 is a good choice on account of the very high productivities it affords at 400xc2x0 C. For the polymerization of HFP/TFE/PMVE, however, where PMVE chain transfer is of prime concern, NF3 which retains good efficiency at 250xc2x0 C., is an excellent choice for initiator. Suitable free radical initiators include NF3, RfNF2, Rf2NF, Rf3N, R1Nxe2x95x90NR1, RfOORf, perfluoropiperazine, and hindered perfluorocarbons of the formula CnF2n+2 wherein each Rf is independently perfluoroalkyl, preferably containing 1 to 20 carbon atoms, hindered perfluoroalkenes of the formula CnF2n, perfluoro(dialkylsulfones) of the formula R1SO2R1, perfluoroalkyl iodides of the formula R1I, perfluoroalkylene diiodides of the formula IRI where the two iodides are not vicinal or geminal, perfluoro(dialkyldisulfides) R1SSR1, and perfluoroalkyl compounds containing nitrogen-sulfur bonds of the formula R12 NSR1, wherein each R1 is independently saturated perfluorohydrocarbyl optionally containing one or more ether groups, isolated iodine, bromine or chlorine substituents, or perfluoroamino groups. By xe2x80x9csaturated perfluorohydrocarbylxe2x80x9d is meant a univalent radical containing only carbon and fluorine and no unsaturated carbonxe2x80x94carbon bonds. The activity of any particular initiator molecule may be readily determined by minimal experimentation.
Preferred initiators are NF3Rf2NF, RfNF2, perfluoropiperazine, perfluoro(dialkylsulfones), i.e., R1SO2R1, and hindered perfluorocarbons. NF3 is an especially preferred initiator. If higher molecular weight polymers are desired, the initiator should preferably not have any groups present in its structure that cause any substantial chain transfer or termination during the polymerization. Such groups usually include, for instance, organic bromides or iodides or carbon-hydrogen bonds.
A useful range of initiator concentration has been found to be about 0.003 to about 0.5 g of initiator/kg monomer, preferably about 0.1 to about 0.3 g/kg. Higher or lower amounts are also useful depending upon the initiator, the monomers, goal molecular weights, process equipment, and process conditions used, and can readily be determined by experimentation. The initiator may be added to the reactor as a solution in the monomer(s).
Various comonomers (III) may be used in the polymerization process, and be incorporated into the polymer. Perfluoro(alkyl vinyl ethers) and perfluorinated terminal alkenes, each optionally substitited with ether, cyano, halo (other than fluorine), sulfonyl halide, hydrogen or ester groups may be used. Also unfluorinated or partially fluorinated olefins or vinyl ethers, optionally substituted as above, may also be used. Useful comonomers include CF2xe2x95x90CFOCF2CF(CF3)OCF2CF2SO2F, ethylene, vinylidene fluoride, CH2xe2x95x90CHO(Cxe2x95x90O)CF3, methyl vinyl ether, CFClxe2x95x90CF2, CH2xe2x95x90CFCF3, CH2xe2x95x90CHCF3, CH2xe2x95x90CHCF2 CF2 CF2 CF3, CH2xe2x95x90CHCF2 CF2 Br, CF2xe2x95x90CFCF2 CN, and CF2xe2x95x90CFCF2 OCF2 CF2 SO2F. Any combination of ethylene, vinylidene fluoride, perfluoroalkylvinyl ethers, perfluorobutylethylene, alkylvinyl ethers and/or vinyl fluoride is preferred.
The properties of the polymer will be affected not only by the overall composition of the polymer, but by the distribution of the various monomer units in the polymer. The instant process yields a polymer in which the monomer units are fairly uniformly distributed in the polymer, which gives polymer with consistent properties. One measure of polymer uniformity is randomness of the monomer units in the polymer. A measure of this is relative amounts of isolated repeat units, diads, triads etc. By diads and triads are meant instances in which two or three repeat units from the same monomer, respectively, occur in the polymer.
Many of the polymers made by the process described herein have relatively small amounts of triads of repeat unit (I), which is of course derived from HFP. Thus, in such polymers, less than 20 mole percent of (I) is in the form of triads, and preferably less than about 15% and more preferably less than about 10%. As would be expected, in polymers with higher amounts of (I), there is a tendency towards higher triad content. The amount of triads in the polymer can be determined by 19F NMR.
Polymers described herein often have a polydispersity of less than 5, preferably less than 4. Repeat units (III) help suppress crystallization and provides for a lower glass transition temperature. Preferred monomers for repeat unit (III) are vinyl fluoride, vinylidene fluoride, perfluoroalkylvinyl ethers, perfluorobutylethylene, alkylvinyl ethers and/or ethylene, with a mixture of at least two most preferred.
Since TFE is considerably more reactive in the polymerization than HFP, an excess of HFP is needed to achieve the desired polymer composition. Typically this also means that at the end of the polymerization, much or all of the TFE will have polymerized, but there will be a considerable amount of unpolymerized HFP. Typically the TFE will be about 1 to 15 mole percent of the total amount of monomer being fed to the process, with the HFP and other monomer(s) (if present) being the remainder. The average residence time is the average amount of time any of the material fed to the reactor actually spends in the reactor, and is a function of the volume of the reactor and the volumetric flow of the process ingredients through the reactor. A preferred residence time is about 20 sec to about 10 min, more preferably about 40 sec to about 2 min. When the process fluids are being added to the reactor, it is preferred if they are preheated just before they enter the reaction to a temperature somewhat less than that of the actual reactor temperature, about 20xc2x0 C. to about 100xc2x0 C. less. This allows one to maintain a uniform constant temperature in the reactor itself, and for the newly added materials to start the polymerization reaction immediately upon entry to the reactor.
The polymer suitable for the practice of the present invention, as so produced, comprises at least about 10 mole percent, preferably about 30 to about 50 mole percent, of monomer units derived from hexafluoropropylene, xe2x80x94CF2xe2x80x94CF(CF3)xe2x80x94. The polymer further comprises 0-50 mole percent, preferably 25-50 mole percent, of monomer units derived from tetrafluoroethylene, xe2x80x94CF2xe2x80x94CF2xe2x80x94. The polymer further comprises about 10-75 mole percent, preferably 10-25 mole percent of one or more monomer units (III) selected from the group consisting of:
xe2x80x94CF2CFXxe2x80x94, xe2x80x94CH2xe2x80x94CHRxe2x80x94, xe2x80x94CH2CF2xe2x80x94
wherein X is H or perfluoroalkoxy having 1-20 carbons, and R is H, alkyl, alkoxy, perfluoroalkyl, or F. Preferably (III) is a repeat unit derived from vinyl fluoride, vinylidene fluoride, alkyl, including fluoroalkyl, vinyl ethers, (perfluorobutyl) ethylene, or ethylene. Most preferably, a combination of at least two of these is employed.
One of skill in the art will appreciate that many compositions are encompassed within the range suitable for the practice of the present invention, and that, particularly at concentrations of HFP below 30 mol-%, not all such compositions will exhibit sufficient amorphous character to be suitable for the practice of the present invention, while others may have little practical value. However, since the repeat units (III) tend to disrupt crystallinity and reduce glass transition temperature while the repeat units derived from TFE (II) tend to increase crystallinity and increase glass transition temperature, the effects of the two groups of repeat units can be traded off against one another. Since some of the repeat units (III), such as vinylidene fluoride, vinyl fluoride, and ethylene, themselves form crystals at high concentrations, while others, such as perfluoroalkyl vinyl ethers or perfluorobutyl ethylene, tend to limit molecular weight, it is found to be particularly effective to employ two or more repeat units (III) in combination, for a total concentration of 10-25 mol-%.
For the purposes of the present invention, the polymer is preferably a low-viscosity liquid at the temperature of the stone surface to which it is applied in order to enhance the uniformity of coating and achieve good coating distribution in a matter of minutes to hours. The desired degree of liquidity is achieved when the glass transition temperature of the substantially amorphous polymer of the invention is below the temperature of application. Additionally, the viscosity of the liquid polymer is determined in part by molecular weight, with lower molecular weight generally associated with lower viscosity. Vinyl fluoride, methyl-vinyl ether, ethylene and similar comonomers each tend to limit the molecular weight of the finished polymer. Thus, one way to tune the polymer viscosity to achieve the optimum for a particular application, is to manipulate the concentrations of those monomers in the polymer. On the other hand, when a solvent is employed during the application of the coating, low molecular weight and low glass transition temperature enhance the solubility of the polymer in the chosen solvent.
In a preferred embodiment, the polymer suitable for the practice of the invention is a liquid, and may be applied to a stone surface directly, without dilution. However, it is preferred to first dissolve the polymer in a solvent to achieve the optimum control over uniformity and thickness of coating.
Coatings formed from the polymer of the invention are particularly useful because of the inherent properties of the polymer, such as lack of crystallinity (polymer is transparent), low surface energy (and hence poor wetting by water or most organic liquids) while exhibiting high surface coverage of stone, low dielectric constant, low index of refraction, low coefficient of friction, low adhesion to other materials, etc.
In the practice of the present invention, one or more of the substantially amorphous fluoropolymers hereinabove described is applied by any convenient method to the surface of the stone which is to be protected from the effects of water and environmental pollutants. It is important that the coating provide a barrier to liquid water with minimal effect on the natural water vapor permeability of the stone. One way of achieving this is to provide a durable coating in as thin a layer as possible on the wall surface of each pore of the stone without actually filling or blocking the pore. This is achieved by using a material of the lowest possible surface tension. Coating materials which exhibit a desirable combination of properties are characterized by pendant groups comprising perfluorinated functional groups in sufficient concentration that the surface presented to incident liquid water such as rainfall is characterized by a high density of the perfluorinated groups and a consequently very low surface tension. In the most preferred embodiment, the polymer suitable for the practice of the present invention comprises 30-50 mol-% of repeat units derived from HFP. The resulting low surface tension is the source of the thermodynamic driving force for complete wetting of the pores in stone as well as for the liquid water repellency of the coated stone. To reduce the kinetic barrier to complete pore wetting, the viscosity should be as low as possible. This represents a particularly desirable attribute of the method of the present invention because the substantially amorphous fluoropolymer employed in the method of the present invention readily forms low viscosity solutions in a number of convenient solvents.
While in no way limiting the scope of the invention, it is estimated that the viscosity of the coating during application of the coating to the stone is preferably less than about 10 Pa-s to achieve optimum coating performance. It will be obvious to one of skill in the art that while it is desirable to employ materials which afford low viscosity solutions, usually associated with low molecular weight or non-polymeric materials, the materials so employed cannot be of such low molecular weight that they evaporate from the stone surface.
It is further preferred that polar groups should be present in the coating material to promote adhesion of the coating material to the stone surface and decrease the tendency of the coating material to continually penetrate to the interior of the stone and reducing surface efficacy in terms of liquid water repellency. Esters, amides, xe2x80x94CH2CF2xe2x80x94 and adjacent xe2x80x94CHCFxe2x80x94 moieties are examples of such adhesion-promoting polar groups.
According to the method of the present invention, the substantially amorphous fluoropolymer can be dissolved in a solvent which acts as a volatile diluent in the spraying operation to afford fast penetration at the early stages of coating while providing a high degree of control over the viscosity, the uniformity of coating and the coating thickness.
Solvents suitable for the practice of the present invention include acetone, methyl-ethyl ketone, ethyl acetate, t-butyl acetate, hydrochlorofluorocarbons, perfluorocarbons. In the most preferred embodiment, the substantially amorphous fluoropolymer of the present invention is dissolved in supercritical CO2 according to the methods described in Carbonell et al., WO 99/19080 or in the alternative in U.S. Pat. Nos. 4,923,720; 5,108,799; 5,290,603; and 5,290,604. To achieve solubility in relatively low pressure carbon dioxide, which is desirable for on-site application of coatings, the combination of the molar ratio of hydrogen to fluorine attached to the polymer backbone, the xe2x80x9cH:F ratioxe2x80x9d, is about 0.3 to about 1.0 as described in U.S. Pat. No. 6,034,170. If the polymer molecular weight is less than about 5000 Da, a somewhat broader H:F ratio can be sustained.
Spray-coating of stone is preferably effected from CO2 solutions of 75 weight % or less polymer at 40 to 70xc2x0 C., 2000 to 4000 psi. To promote polymer absorption into the stone it might also be preferable to add up to about 40 weight % acetone, t-butyl acetate, Oxol 100 (4-chlorobenzotrifluoride), or other such environmentally friendly diluents to the substantially amorphous fluoropolymer.
It will be understood by one of skill in the art that numerous chemical compounds have been identified which may serve as the supercritical fluid for the substantially amorphous fluoropolymer coating composition of the invention. However, CO2 is by far the preferred compound because of the low cost, low toxicity, ready formation of a supercritical fluid, and low environmental impact.
The substantially amorphous fluoropolymer component of the coating composition is generally present in amounts ranging from 1 to 80 weight percent, based upon the total weight of the coating composition. Preferably, the substantially amorphous fluoropolymer component should be present in amounts ranging from about 15 to about 70 weight percent on the same basis.
The supercritical fluid diluent should be present in such amounts that a liquid mixture is formed that possesses such a viscosity that it may be applied as a liquid spray. Generally, this requires the mixture to have a viscosity of less than about 300 centipoise at spray temperature. Preferably, the viscosity of the mixture of components ranges from about 5 centipoise to about 150 centipoise. Most preferably, the viscosity of the mixture of components ranges from about 10 centipoise to about 50 centipoise.
The supercritical carbon dioxide fluid is most preferably present in amounts ranging from about 30 to about 85 weight percent on the total compositional weight, thereby producing a mixture having viscosities from about 10 centipoise to about 50 centipoise at spray temperature.
It is not necessary to form a preliminary solution or dispersion of the preferred substantially amorphous fluoropolymer composition in order to form a low-viscosity solution or dispersion suitable for mixing with the CO2. It is however optional to add a third component to the coating composition of the invention, the third component comprising one or more organic solvents employed for the purpose of improving viscosity control during spraying and xe2x80x9claydownxe2x80x9d of the coating material on the stone.
The organic solvents suitable for the practice of the most preferred embodiment of the invention generally include any solvent or mixture of solvents that is miscible with CO2, is a good solvent for the substantially amorphous fluoropolymer, and is fugitive at the temperature at which the coating is being applied to the stone, normally at temperatures of about 0xc2x0 C. or above. Preferably, the solvent is also environmentally friendly. Suitable organic solvents include acetone, methyl-ethyl ketone, ethyl acetate, t-butyl acetate, hydrochlorofluorocarbons, and perfluorocarbons with acetone, methyl-ethyl ketone, ethyl acetate and t-butyl acetate preferred.
The coating composition of the invention is sprayed onto a substrate to form a liquid coating thereon by passing the liquid mixture under pressure through an orifice into the environment of the substrate to form a liquid spray.
Spray orifices, spray tips, spray nozzles, and spray guns used for conventional airless and air-assisted airless spraying of coating formulations such as paints, lacquers, enamels, and varnishes, are suitable for spraying the coating composition of the present invention. The spray pressure used in the practice of the present invention is a function of the specific coating formulation. In the case of supercritical fluid solutions, the minimum spray pressure is at or slightly below the critical pressure of the supercritical fluid. Generally the pressure will be below 5000 psi. Preferably, the spray pressure is above the critical pressure of the supercritical fluid and below 3000 psi. If the supercritical fluid is supercritical carbon dioxide fluid, the preferred spray pressure is between 1070 psi and 3000 psi. The most preferred spray pressure is between 1200 psi and 2500 psi.
The spray temperature used in the practice of the present invention is a function of the coating formulation. The minimum spray temperature is about 31xc2x0 C. The maximum temperature is determined by the thermal stability of the components in the liquid mixture. The preferred spray temperature is between 35xc2x0 C. and 90xc2x0 C. The most preferred temperature is between 45xc2x0 C. and 75xc2x0 C. Generally liquid mixtures with greater amounts of supercritical carbon dioxide fluid require higher spray temperatures.
One of skill in the art will recognize that the method of the present process, while specifically directed to the protection of stone, may equally be employed to apply coatings to a variety of substrates. Examples of suitable substrates include but are not limited to metal, wood, glass, plastic, paper, cloth, ceramic, masonry, stone, cement, asphalt, rubber, and composite materials.
Through the practice of the present invention, coatings may be applied to substrates in thicknesses of from about 0.5 to 100 micrometers. Preferably, the coatings have thicknesses of from about 1.0 to about 15 micrometers, while most preferably, their thicknesses range from about 1.5 to about 10 micrometers.
The method of the present invention provides a considerable benefit in that the substantially amorphous fluoropolymer coating may be readily removed using common solvents such as acetone, methyl-ethyl ketone, ethyl acetate or t-butyl acetate, if it should be deemed desirable at some point in time following the application thereof.
The coatings on stone produced by the practice of the present invention are highly beneficial to the purpose of protecting the stone from environmental degradation. Two key attributes which are indicative of susceptibility to weathering are water absorption, typically by capillary action through the porous stone structure, and water vapor permeation rate. It is highly desirable that the water absorption of normally highly absorbent stone be reduced by as large a factor as possible, while water vapor permeability, normally high as well, be maintained at a high level. The coated stone of the present invention provides high levels of water vapor permeability by virtue of the thin coatings which are found to be effective in providing the desired high resistance to water penetration.
The method of the present invention and the properties of the coated stone compositions provided thereby are further illustrated in the following specific embodiments.